An electroluminescence (referred to below as EL) element is known as a light emitting element which uses an electroluminescence phenomenon. An EL element can emit light in colors of various wavelengths by selecting a light emitting material which forms a light emitting layer, and their application to display devices and lighting fixtures is progressing. In particular, an EL element which uses an organic material as a light emitting material is attracting attention.
In a display device using an EL element, an EL element as a light emitting element and a switching element which performs light emitting control of the EL element are arranged in each pixel arranged in a matrix shape above a substrate. In addition, by controlling the ON/OFF of a switching element for each pixel, it is possible to display an optional image across an entire display area.
Two types of display devices described above are known, a top emission type and bottom emission type. A top emission type is a method in which light emitted by an EL element is extracted to the opposite side of a first substrate, that is, second substrate (sealing substrate) side, formed with a pixel circuit, and a bottom emission type is a method in which light emitted by an EL element is extracted to an array substrate side. In particular, since a bottom emission type is arranged with a pixel electrode above a region formed with a pixel circuit and can effectively use the majority of a pixel electrode as a light emitting region, a bottom emission type is effective in being able to secure a high aperture ratio of a pixel.
In a top emission type EL display device, it is necessary to extract light that has passed through a common electrode (cathode) which becomes a pair with a pixel electrode (anode). Therefore, in a top emission type, MgAg (Magnesium-Silver Alloy,) which is formed into a thin film so as to sufficiently allow light to pass through, is used as the common electrode, and a transparent conductive film of ITO (Indium Tin Oxide) or IZO (Indium Zinc Oxide) is often used.
Among these, an ITO or IZO transparent conductive film has a higher resistance compared to a metal film. Therefore, when the size of a pixel in a display device increases, the effects of a drop in voltage due to the resistance component of a transparent conductive film increases which leads to problems such as differences in luminosity in a screen.
A structure in which auxiliary wiring is arranged above a common electrode after forming a common electrode is disclosed as a method for solving the phenomenon described above in Japanese Laid Open Patent 2009-276721. The above described technology aims to achieve low resistance of a common electrode by arranged auxiliary wiring comprise from a metal film above a common electrode. In addition, efforts are made for the auxiliary wiring to not cause a decrease in the aperture ratio of a pixel by arranging the auxiliary wiring above a bank. Furthermore, a bank is a component is formed from an insulation material such as a resin which sections a pixel.
In the technology described above, it is difficult to align a bank and auxiliary wiring in the case where high definition of a display device has progressed, and when alignment precision is poor, the auxiliary wiring is sometimes formed within a pixel. As a result, the auxiliary wiring may block the light emitted from an EL element leading to a decrease in an aperture ratio.
Therefore, an aim of one embodiment of the present invention is to provide a technology for forming auxiliary wiring above a common electrode with high alignment precision.
In addition, an aim of one embodiment of the present invention is to provide a high definition display device securing both low resistance of a common electrode and a sufficient aperture ratio using auxiliary wiring.